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Thermocouple measurements _ instrument

by:JVTIA     2020-10-28
Thermocouple material used at present, there are mainly platinum, copper, nickel, etc. , using these materials mainly is that they are in common temperature period of temperature and the resistance is the ratio of the linear relationship, we here mainly introduce platinum thermocouple thermometer. Platinum is a precious metal, its physical and chemical performance is stable, especially the oxidation resistance ability is very strong, it is easy to be purified, good manufacturability, can be made into very fine platinum, and copper, nickel and other metals, compared with high resistivity, high reproducibility, is a kind of ideal material of thermocouple, defect is temperature coefficient of resistance is small, work is brittle in the reducing medium, the price is more expensive. The purity of platinum usually used resistance ratios for: W ( 100). = R100 / R0 R100 indicates the resistance value of 100 ℃; R0 indicates the resistance value of 0 ℃ according to the IEC standard, adopting W ( 100). = 1. 3850 initial resistance value R0 = 100 Ω ( R0 = 10Ω) Platinum resistance for industrial use standard platinum thermocouple, R0 = 10 Ω platinum thermocouple thermometer with thicker wire, mainly used in measuring temperature above 600 ℃. Platinum resistance of resistance and temperature equation for a segmented equation: Rt = R0 [ 1+At+Bt2+C( t - 100℃) t3] T says in - 200-0 ℃ Rt = R0 ( 1+At+Bt2) T says in 0 ~ 850 ℃ solution of this equation, can according to the resistance temperature is known, but in the actual work, can check thermocouple indexing table according to the resistance temperature is determined. According to the prescribed standards, platinum thermocouple is divided into class A and class B, class A temperature allowed error of plus or minus ( 0. 15℃+0. 002 t | |) , class B temperature allowed error of plus or minus ( 0. 3℃+0. 005 t | |) 。 Field use of thermocouple are generally thermocouple, it is by the thermocouple protection tube, insulating material, body composition, thermocouple protection tube body and welding together, middle filled insulation materials, so that is good for protect thermocouple, impact resistance, shock resistance, corrosion resistance. Three-wire system platinum thermocouple measurement: platinum thermocouple two wire, three wire system, four wire several, two wire system in the measurement error is bigger, has not use, now commonly used in industry is three wire system, lab with general of the four wire system. Here introduces three wire system under platinum thermocouple wiring. Three-wire system platinum thermocouple is at the end resistance of a parallel a c side, so as to realize the resistance leads to a, b, c three terminals, in this way, the b wire is introduced into measuring the resistance of the wire itself, can be compensated by c wire, make lead resistance do not vary with temperature and introduce the effect of lead resistance error decreases a lot. Three-wire system platinum thermocouple, in secondary instrument, all have variable resistance of the bridge, based on the coordinate of platinum thermocouple range is different, the bridge of the secondary instrument platinum thermocouple fine-tuning, can make more precise measurement of the thermocouple wire 1, two wire there are three main ways: in the thermocouple at both ends of the connection to elicit resistance signal way is called a wire two wire: this kind of wire method is very simple, but because the wires are inevitably exist lead resistance r, r size and material of the wire and the length of the factors, so this method is only applicable to lead low measurement precision of 2, three-wire system: at the root of the thermocouple connection at one end of a wire, the other end connected to the two lead way called three wire system, the way which is equipped with a bridge, can be better to eliminate the influence of the lead resistance, is commonly used in industrial process control. 3, four wire: on both ends of the thermocouple root way each connecting two wire called a four wire system, including two fuses for thermocouples provide constant current I, u r into a voltage signal into, again through the other two root lead to guide u to secondary instrument. Is this way of lead can completely eliminate the resistance of the lead, it is mainly used for high precision temperature detection. Thermocouple temperature scoring methods: new industrial platinum resistance thermometer is a kind of widely used temperature measuring instrument. For a long time, domestic and foreign relevant technical standards or technical specifications of CVD equation is widely used in the calculation method of calibration points to degrees. But the verification by using CVD equation, dividing the industrial platinum resistance thermometer is not high, stability, low accuracy and uncertainty is bigger, cannot be used as a transfer standard. Therefore, the majority of industrial temperature measurement or request is not high in the field of laboratory using high precision standard platinum resistance thermometer as traceability standard, but the actual industrial temperature field due to various constraints, cannot use standard platinum resistance thermometer, makes the temperature dissemination and traceability in these places cannot be achieved, can not carry out the actual measurement calibration work. The industrial platinum thermocouple thermometer to check the feasibility of dividing, and widely used of equation of CVD temperature - Resistance relationship compared the calculation results, which shows the differences between, forming a precision industrial platinum resistance thermometer as the standard for transmission way and the method. Based on different models, different manufacturers more than the industrial platinum thermocouple temperature in different area, respectively, to carry out research and analysis, the result of the experiment, the data curve of each thermometer using two different methods measuring error caused by dividing. Experiments show that ITS - 1990 international temperature scale of interpolation method for the industrial platinum thermocouple thermometer is feasible, and CVD equation used for the industrial platinum resistance checks, compared to the calculation method of dividing has good accuracy and consistency. Previously, Italy and Canada's national metrology technical agencies adopt international temperature scale interpolation formula research work of industrial platinum resistance dividing method. Industrial resistance temperature measurement accuracy and stability of traditional means in components, packaging technology, production process and purity; From calculation method gives a new way of thinking, for precision platinum resistance and industrial thermocouple temperature dissemination and the improvement of the traceability system laid the foundation, can be widely used in the field of industrial platinum thermocouple temperature measurement.
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